474 GEOLOGICAL SURVEY OF IOWA. 



of any established groups of the Silurian and Devonian 

 periods. 



An examination of the fossils from the geological horizon 

 of the Hamilton and Chemung groups of New- York ( and 

 which is in fact a continuation of the same beds) shows, 

 that while there are many species identical with those of 

 the same group in its eastern localities, there are a still 

 larger number of new species ; and which, since the physi- 

 cal conditions of the mass were not essentially different, can 

 only be attributed to the influence of geographical distance. 

 Having partially the means of illustrating in the fossils the 

 changes which have taken place in the fauna of these epochs, 

 I have given, to some extent, both the already known and 

 the new species. 



The Hamilton and Chemung groups, after being traced 

 almost continuously from the western slopes of the Appala- 

 chians, is recognized in the Mississippi valley, both by 

 lithological features and by fossil remains ; and we take up 

 again in Iowa our investigations of the fossils of these groups, 

 after an interval of nearly fifteen degrees of longitude. 



Among the Carboniferous limestones, the Burlington lime- 

 stone has given ample materials for the illustration of its 

 ancient fauna. The Keokuk limestone has also proved pro- 

 lific of characteristic fossils ; while the St. Louis limestone 

 has afforded, comparatively, so few species that I have been 

 constrained to go beyond the limits of the State for many 

 of the characteristic examples of its fossils. The Kaskaskia 

 limestone, so well developed at Kaskaskia and thence along 

 the river bottom to Chester and as far as Fountain bluff, is 

 unknown up to the present time within the limits of the 

 State of Iowa. I have, nevertheless, felt it my duty to illus- 

 trate in some measure its fossil remains, in order to prove 

 its distinctness from the Keokuk limestone, with which it 

 has heretofore been erroneously identified as the "Archi- 

 medes limestone". The fossils selected for this purpose are 



